Improvement of the thermal efficiency of an engine is desired for the purpose of improving fuel consumption. Patent Document 1 describes an invention for providing a film having low thermal conductivity and low heat capacity, preventing such problems as peeling and dropping off, and which has high durability and reliability for use in an engine combustion chamber. In the invention according to Patent Document 1, an anodic oxide coating with a film thickness ranging from more than 20 μm to 500 μm and a porosity of 20% or more is formed on the inner wall constituting an engine combustion chamber for improving the thermal efficiency of the engine.
Patent Document 2 describes an invention for providing an internal combustion engine having structural members with improved thermal insulation performance. In the internal combustion engine according to Patent Document 2, a heat insulating material is provided adjacent to the inner wall of an exhaust gas through channel, wherein a heated working medium (exhaust gas) flows along the through channel formed using the heat insulating material. The heat insulating material is laminated in a state such that spherical mesoporous silica particles with an average diameter of 0.1 to 3 μm agglomerate via a bonding material. A number of meso pores with an average diameter of 1 to 10 nm are provided on each of the spherical mesoporous silica particles.